The present invention relates to a glass whose transmittance to ultraviolet rays and visible rays is such that the glass loses transmittance upon exposure to ultraviolet rays and recovers transmittance at a very slow rate in a dark place at room temperature, at an accelerated rate upon exposure to long-wavelength rays and at a very rapid rate upon exposure to heat. Because of its ability to lose and regain transmittance, the glass according to the present invention is referred to as "reversibly photosensitive glass".
Reversibly photosensitive glasses have been known heretofore and it is a property of such glasses to vary in transmittance to light in a reversible manner in accordance with the quantity of sunlight impinging thereon. Thus, such glasses have been used for sunglasses, for example. Since reversibly photosensitive glasses have an additional advantageous feature of not requiring aftertreatments such as image development, studies on the feasibility of such glasses in applications to optical memories and other optical data processing devices are now under way.
The borosilicate glass precipitating a minute amount of silver halide ["Photochromic Silicate Glass Sensitized by Silver Halides", W. H. Armistead et al, Science, Vol. 144, No. 3615, 150-154 (1964)] is but one example of such glasses which have heretofore been put to practical use. Since these glasses have been developed mainly for use in sunglasses, they are characterized by the property of assuming a dark color and accordingly losing transmittance upon exposure to darkening lights and, upon exposure to normal indoor conditions of room temperature, quickly losing the dark color and recovering their transmittance. This characteristic property, however, turns out to be a disadvantage when the glasses are to be used such as in optical memories which are primarily aimed at preserving images and figures for a long period of time. Furthermore it is desired that the glasses have high darkening sensitivity.
An object of this invention is to provide a reversibly photosensitive glass which recovers the lost transmittance at a very slow rate in a dark place at room temperature and responds with high sensitivity to the darkening light.
Another object of this invention is to provide a reversibly photosensitive glass which readily recovers transmittance upon exposure to long-wavelength rays or to heat.